Advances in the field of quantum cryptography have led to the development of methodologies for the secure distribution of a cryptographic key over a quantum channel. This is known as “quantum key distribution” or QKD. Specifically, by exploiting the properties of a quantum channel, one can devise protocols that allow two communicating parties (referred to in cryptography parlance as “Alice” and “Bob”) to detect when the quantum channel has been intercepted or otherwise tampered with by an intermediate party (referred to as “Eve”). Thus, as long as no such interception or tampering has been detected, Alice and Bob can rest assured that their cryptographic key will have been distributed in complete security over the quantum channel. The cryptographic key is then used by Alice and Bob in subsequent encryption (over a classical channel) of possibly larger amounts of information requiring secure transmission.
A specific example of a quantum channel is an optical fiber, which transports “pulses”, each of which contains zero or more photons. However, when transmitting photons over long distances, they may become so severely attenuated as to render them undetectable by Bob's receiver. Hence, when performing QKD over long distances, it becomes necessary to install repeaters every several kilometers or so, whose function it is to detect photons transmitted by a previous “hop” and to re-transmit them to the next hop. Several types of repeater architectures have been devised to meet the needs of long-haul QKD.
A first type of repeater architecture utilizes conventional quantum reception and transmission devices at each hop, while relying on the so-called BB84 protocol for communication over the quantum channel spanning between adjacent hops. For details about the BB84 protocol, the reader is referred to C. H. Bennett and G. Brassard, “Quantum Cryptography: Public Key Distribution and Coin Tossing”, Proceedings of IEEE International Conference on Computers Systems and Signal Processing, Bangalore, India, December 1984, pp. 175-179, hereby incorporated by reference herein.
Unfortunately, by virtue of the base mismatch phenomenon that is inherent to the BB84 protocol, an average of 50% of the data that is transmitted from one hop to the next is forfeited at that next hop. As a result, with N repeaters placed between Alice and Bob, the loss exclusively attributable to use of the BB84 protocol between Alice and Bob will be 1/2(N+1). By way of example, a system that has three repeaters (i.e., four hops) and which uses the BB84 protocol between hops will allow no more than about 6 percent of an original amount of data to be transmitted securely from Alice to Bob. Clearly, this degree of loss is undesirable and becomes even more so as the number of hops grows.
A second type of repeater architecture contemplates the use of devices with a so-called “quantum memory”, which attempts to capture photons without altering their state (i.e., without detecting them). The photons captured at one hop are then re-transmitted to the next hop by ejecting them from the quantum memory. However, this technology is currently still considered experimental and not commercially viable, as its sensitivity to extraneous factors as well as its ability to function at high data rates has not yet been fully investigated. Moreover, it is an expensive technology and thus, overall, quantum memory devices are not considered to provide a practical solution for long-haul QKD.
Therefore, a need clearly exists in the industry for an improvement over existing methods and systems used for communicating over a quantum channel.